VNAV Stalls/Loses Altitude during Cruise - Possible Answer

[Guest hankaaron]

I had the same loss of altitude as reported in various other POST. The 747-400 would begin to stall during cruise on westbound flights. The stalls occurred as the aircraft attempted to retain altitude by increasing the degree of nose up attitude.I found that verifying the the CG Trim in the FMC prevents the stalls from occurring. The CG (Center of Gravity) is listed right under the takeoff refs. See if this helps...Aaron Morris.

[Guest D17S]

That's interesting. Can you elaborate? When you say 'verify the CG trim' . . what exactly do you mean? The real airplanes have a system called "auto stab trim" that automatically trims the stab for various elevator positions. The stab trims so the elevator can run faired (rather than sticking up or down into the airstream) to maintain a required nose up or down attitude. A shifting CG (fuel burn) would be a reason the elevator (and then the stab) would have to move maintain a pitch attitude. But this should all be automatic. In the old -200s there would be lights that would come on to announce stab trim had failed to move on command. I guess EICAS is the deal now. Any annumciations about autostab trim inop? Sounds like something is going wrong here. (BTW, autothrottles all hooked up and working OK? That's been the main problem lately)

[Tabs 2,790]

What is your ZFW, how much fuel do you have on board, and what does the VNAV cruise page show your optimal and max altutudes as?

[emvaos 13]

Also what else apart from the 747 are you running concurrently ?If VNAV is engaged the aircraft cannot/wiil not stall. It has a stick pusher modelled. It will automatically drop the nose unconditionally until measured angle of attack is safe. You have to understand that wind shifts of the order of 10-20-50 knots happening within 3 seconds do not happen in real life and if they do the amount of turbulence generated would be enough to brake the plane. You also need to know that per degree pitch the engines need to spool up an additional 4% N1 to maintain the same speed. Accordingly at high altitudes you should never ever allow pitch to exceed 5 degrees. Example : at 350 tons, M0.84, FL350, pitch is 3.2 degrees and N1 required approx 96.4. At 300 tons same Mach, same altitude, required N1 is down to 91.6 for pitch 2.5 degrees. At 250 tons same Mach same altitude pitch is only 1.8 degrees and required N1 is now only 88.4. All this due to the exorbitant drag difference caused by slight pitch differences. When you lose 20 knots speed due to a sudden wind shift in FS, you rob the wings of tons and tons of lift (as if the plane weighs tons and tons more).Unfortunately we talk about an extremely accurate performance model coupled to mickey-mouse wind modelling. My best advice is get a utility that allows wind smoothing such as a registered version of FSUIPC (in v3.51 it is noted that finally this issue is fixed by Pete). ASV has also such an option --- if you have it click it !Stab trim features : This is the only sim that the AP (technically the Flight Control Centre) will primarily use elevators to adjust pitch (not just trim) as per real unit. In fact in the VC you can watch the control column "bob" back and forth during pitching manouvers (although in a bit of an exaggerated manner). The FCC will continuously adjust stab trim in a "measured manner" during pitching action to zero elevator position. Also FCC will automatically add positive trim bias during autoland to facilitate flare and go around. For manual flight "control column cut off" is also modelled, i.e, if you push the "control wheel" it will not allow trim UP and vice versa. Furthemore, for manual flight there exists a stab trim "limiter" with action proportional to pitch, CG and IAS (that's what you refer to Sam I guess...)Best,Vangelis===================================== E. M. Vaos Precision Manuals Development Group www.precisionmanuals.com=====================================

[Guest hankaaron]

Here are my repsonses to the previous inquiries."That's interesting. Can you elaborate? When you say 'verify the CG trim' . . what exactly do you mean?"There is a setting in the Take-off page of the FMC where Center of Gravitiy is set and confirmed. I simply confirm the setting and just to the right, another value appears automatically for trim setting. "What is your ZFW, how much fuel do you have on board, and what does the VNAV cruise page show your optimal and max altutudes as?"ZFW was about 488 thousand pounds, the failure occurred pretty earlier during the cruise phase so I had a little less than 300 thousand pounds of fuel."If VNAV is engaged the aircraft cannot/wiil not stall. It has a stick pusher modelled. It will automatically drop the nose unconditionally until measured angle of attack is safe. You have to understand that wind shifts of the order of 10-20-50 knots happening within 3 seconds do not happen in real life and if they do the amount of turbulence generated would be enough to brake the plane."The failure also happened with wind manually set to zero.

[Guest D17S]

Ahh,-- kinda of, You are confirming the CG number just to verify that you are aware of what the trim setting should be. It's like a 'sign here' line. You then have to set the trim manually prior to take off as part of your pre-flight setup.If the stab trim is set wrong you might certainly have a heck-of-a-time with takeoff and a manual climb. However if the auto stab trim system is working, this should not matter once you have the AP hooked up and give the system a minute to get things sorted out. Here